Framework interacting with probe molecules

Several authors used diffuse reflectance UV-Visible spectroscopy to probe the local environment of Ti sites [136-138,140,141,143,147]. Sayari et al. [137,141,147] found that both Ti-HMS and Ti-MCM-41 exhibit an absorption band at 220 ran with no indication of a band at 330 ran characteristic of anatase [148]. There was however a weak shoulder at 270 nm, particularly for Ti-iich samples. Conna et al. [140] also found a band at 205-220 nm and a shoulder at 270 nm. The absence of anatase was also confirmed by the absence of the characteristic 140 cm Raman band [138,149]. The band at 220 nm was attributed to isolated framework Ti species in interaction with water molecules [150]. The 270 nm band was assigned to partially condensed hexacoordinated Ti species belonging to a silicon rich amorphous phase [155]. 

The interaction of probe molecules with acidic OH groups is also studied, " even though the detail is not described in this report. Moreover, the spinning sidebands analysis in H MAS NMR offers the information on the distance between the bridging OH groups and aluminium ions in the zeolite framework. - ... 

The basicities of alkali metal-form (M ) zeolites have been characterized with pyrrole (77,78) and furan (79) as probe molecules. The spectra, supported by calculations, suggest that in both cases there are two interactions. For pyrrole, there is the expected N-H- -0- framework hydrogen bonding and also a interaction. For furan, there is a and also C-F1---0- framework hydrogen... 

Water has been widely used as a probe molecule for the characterization of zeolites, especially of those with a high aluminium content [9]. Water adsorption on hydrophilic zeolites has been used to measure their pore volume, and it has been shown that the amount of water adsorbed is a linear function of the aluminium content [10]. Additionally, water adsorption is also highly sensitive to the nature, valence and accessibility of extra-framework cations [11]. Immersion calorimetry allows for the measurement of the degree of interaction between the zeolite and water, and this can be compared with the interaction between the zeolite and other molecules with different polarity. In this way, the polar character of the zeolite surface can be assessed. 

The interaction between the adsorption sites of a zeolite and a probe molecule perturbs the normal vibrations of the molecule and leads to frequency shifts compared to the gas phase which can be interpreted in terms of the strength and nature of the interactions with the adsorption sites of the zeolite. INS is advantageous for observing such interactions for hydrogenous molecules [116,117]. In the infrared spectra the out-of-plane bending modes and the ring vibrations coupled with CH in-plane bending are obscured by the zeolite framework modes below 1300 em . ... 

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